To accelerate the development of nanotechnologies for drug and gene delivery, it is highly desired to construct nanoparticles with imaging capabilities so that the process of delivery and release can be monitored and quantified with a medical imaging modality. Although there have been successful preclinical studies that showed the possibility of such monitoring, there is clearly a gap between the demand for clinically-compatible imaging methods to monitor the nanoparticle-mediated drug delivery and release and the current nanoparticle tagging strategies, which often require the use of metallic or radioactive contrast agents. To address this gap, bioorganic molecules have recently been developed as non-labeled (i.e., not radioactive, and not paramagnetic- or super-paramagnetic-) tracers that can be detected through Chemical Exchange Saturation Transfer (CEST) MRI technology. The long-term goal of our research is to exploit bioorganic drugs or drug analogues as CEST MR imaging contrast agents for tagging of nanoparticles, and subsequently translating this new technology to clinical applications. As an initial demonstration of such a principle, this application aims to develop, without the need for additional imaging probes, a sensitive CEST MRI-trackable liposome system to monitor tumor-targeted delivery of 5-FC, and consequently, to predict the therapeutic effect of cytosine deaminase (CD)/5-FC gene therapy. The central hypothesis is that the CEST signal carried by 5-FC can be directly used to detect 5-FC encapsulating liposomes, thus enabling the monitoring and potential quantification of drug-carrying nanoparticles with CEST MRI. Guided by strong preliminary data, this hypothesis will be tested through three specific aims: 1) To develop a sensitive CEST MRI-trackable liposome encapsulating prodrug for 5-FC; 2) to assess antitumor effects of liposome-mediated prodrug delivery; and 3) to monitor liposome-mediated prodrug delivery using CEST MRI. Under the first aim, starting from an already proven liposomal formulation with sufficient CEST detectability, we will optimize the liposomal formulation to obtain a system with improved CEST sensitivity as well as favorable characteristics for drug delivery. Under the second and third aims, we will apply the self-trackable liposome system on experimental animals, assess the antitumor effects, and quantify the enhanced drug delivery with CEST MRI. These aims are expected to result in a translatable nanotechnology to obtain tumor-targeted prodrug delivery in CD suicide gene therapy that can be monitored by non-invasive CEST MRI. The innovation of this proposed research lies in a non-labeled approach to tag nanoparticles based on the drugs they carry. The proposed research is significant, because it is expected to shift the paradigm of the tagging strategy for MR imaging of nanoparticle-mediated drug delivery from metallic agents to bioorganic drug analogues. Ultimately, such a new multifunctional nanoparticle system has the potential to boost the development of an image-guided nanoparticle system for gene and drug delivery, either as an 'effect enhancer' for existing therapies or as an initiator of new therapies